Sphingosine-1-phosphate (S1P) is a bioactive molecule with potent effects on multiple organ systems. Saba, J. D. and Hla, T. Circ. Res. 94:724-734 (2004). Although some believe the compound is an intracellular secondary messenger, its mode of action is still a subject of debate. Id. Indeed, even its metabolism is poorly understood. Hla, T., Science 309:1682-3 (2005). Researchers currently believe that S1P is formed by the phosphorylation of sphingosine, and degraded by dephosphorylation or cleavage. Its cleavage into ethanolamine phosphate and a long-chain aldehyde is reportedly catalyzed by S1P lyase. Id.; Pyne & Pyne, Biochem J. 349:385-402 (2000).
Sphingosine-1-phosphate lyase is a vitamin B6-dependent enzyme localized in the membrane of the endoplasmic reticulum. Van Veldhoven and Mannaerts, J. Biol. Chem. 266:12502-12507 (1991); Van Veldhoven and Mannaerts, Adv. Lipid. Res. 26:69 (1993). The polynucleotide and amino acid sequences of human S1P lyase and its gene products are described in PCT Patent Application No. WO 99/16888.
Recently, Schwab and coworkers concluded that a component of caramel color III, 2-acetyl-4-tetrahydroxybutylimidazole (THI), inhibits S1P lyase activity when administered to mice. Schwab, S. et al, Science 309:1735-1739 (2005). While others have postulated that THI exerts its effects by a different mechanism (see, e.g., Pyne, S. G., ACGC Chem. Res. Comm. 11:108-112 (2000)), it is clear that administration of the compound to rats and mice induces lymphopenia and causes the accumulation of mature T cells in the thymus. See, e.g., Schwab, supra; Pyne, S. G., ACGC Chem. Res. Comm. 11:108-112 (2000); Gugsyan, R., et al., Immunology 93(3):398-404 (1998); Halweg, K. M. and Büchi, G., J. Org. Chem. 50:1134-1136 (1985); U.S. Pat. No. 4,567,194 to Kroeplien and Rosdorfer. Still, there are no known reports of THI having an immunological effect in animals other than mice and rats. Although U.S. Pat. No. 4,567,194 alleges that THI and some related compounds may be useful as immunosuppressive medicinal agents, studies of the compound in humans found no immunological effects. See Thuvander, A. and Oskarsson, A., Fd. Chem. Toxic. 32(1):7-13 (1994); Houben, G. F., et al., Fd. Chem. Toxic. 30(9):749-757 (1992).